BufferHubQueue: ClearAvailable on consumer import

This fixes the issue that ConsumerQueue keeps receiving
|OnBufferAllocated| events when using UDS based libpdx. It also hardens
error handling during consumer imports.

Bug: 34690159
Test: Build, flash, buffer_hub_queue-test pases
Change-Id: I4f739a2869df851ed836f39122b3eba95697f3f6